1. Field of the Invention
This invention relates to actuator devices for movable member manipulation, used in belt-type continuously variable transmissions of automobiles and industrial machines and the like, for example.
2. Description of the Related Art
Belt-type continuously variable transmissions may be used in some automobiles, industrial machines, etc. One such belt-type continuously variable transmission comprises a pulley (primary pulley) located on a rotating shaft on the driving side, a pulley (secondary pulley) on a rotating shaft on the driven side, and an endless belt stretched around these pulleys.
Each of the pulleys includes a fixed pulley disk for use as a fixed member and a movable pulley disk as a movable member, which are opposed to each other. The fixed pulley disk is provided integrally on the rotating shaft. The movable pulley disk is mounted on the rotating shaft so as to be nonrotatable and movable in the axial direction. The movable pulley disk is moved in the axial direction of the rotating shaft by means of an actuator. This movement changes the distance between the fixed pulley disk and the movable pulley disk, that is, pulley width.
In order to change the ratio between the respective rotational speeds of the driving-side rotating shaft and the driven-side rotating shaft, the pulley width on the driving side and the pulley width on the driven side are synchronously changed in opposite directions. In shifting the secondary pulley to the speed increasing side, for example, the pulley width of the primary pulley is reduced, while the pulley width of the secondary pulley is increased. If this is done, the endless belt moves toward the outer periphery of the primary pulley, and the endless belt moves toward the center of the secondary pulley.
A ball screw mechanism is used as the actuator that moves the movable pulley disk in the axial direction of the rotating shaft. The ball screw mechanism is provided with a cylindrical screw shaft on the outer periphery of the rotating shaft and a nut member that is threadedly engaged with the outer periphery of the screw shaft with a plurality of balls between them. The screw shaft is fixed to a transmission case for use as a supporting member. The nut member is rotatable with respect to the movable pulley disk and movable integrally with the movable pulley disk in the axial direction of the rotating shaft.
Ball screw grooves are formed individually on the outer peripheral surface of the screw shaft and the inner peripheral surface of the nut member. The balls are arranged along the ball screw groove of the screw shaft and the ball screw groove of the nut member between these ball screw grooves. If the nut member rotates, the balls roll along the ball screw grooves, and the nut member moves in the axial direction of the screw shaft.
One end portion of the nut member is fitted in a boss portion of the movable pulley disk by means of a bearing. The nut member is rotatable with respect to the movable pulley disk and movable integrally with the movable pulley disk in the axial direction of the rotating shaft.
If the nut member is rotated by means of a drive source such as a motor, the nut member moves integrally with the movable pulley disk in the axial direction of the rotating shaft. The pulley width between the movable pulley disk and the fixed pulley disk is changed by this movement.
In the power transmission apparatus in which the endless belt is passed around and between the movable pulley disk and the fixed pulley disk, a component of force based on the tension of the belt always acts in the direction (first axial direction) to move the movable pulley disk away from the fixed pulley disk. Thus, the nut member moves in the first axial direction and in a second axial direction opposite to it in a manner such that it is subjected to a load in the first axial direction. If the nut member moves in the first axial direction, the distance from the movable pulley disk to the fixed pulley disk increases. If the nut member moves in the second direction, on the other hand, the distance from the movable pulley disk to the fixed pulley disk decreases.
FIG. 4 is an enlarged view of a principal part of a conventional ball screw mechanism. Ball screw grooves a1 and b1 are formed on the outer peripheral surface of the screw shaft a and the inner peripheral surface of a nut member b, respectively. A plurality of balls c are rollably accommodated between the ball screw grooves a1 and b1.
If the nut member b rotates with respect to the screw shaft a, the balls c roll in the ball screw grooves a1 and b1 as they spirally move along the screw grooves a1 and b1. Thereupon, the nut member b moves in the axial direction of the screw shaft a.
As shown in FIG. 4, the cross section of each of the conventional ball screw grooves a1 and b1 has the shape of a Gothic arch. Each ball c is in contact with four points A, B, C and D on surfaces on the opposite sides of groove bases d and e of the ball screw grooves a1 and b1. The nut member b moves with respect to the screw shaft a in the first axial direction or the opposite second axial direction, depending on its rotating direction. During this movement, the ball c is pressed against the points A, B, C and D of the ball screw grooves a1 and b1, so that the load of power transmission acts on these points. Conventionally, therefore, the ball screw grooves a1 and b1 are entirely finished by precise polishing or grinding, in order to reduce friction that is caused by contact with the ball c.
In the ball screw mechanism in which the load in the first axial direction acts always on the nut member, as in the belt-type continuously variable transmission, each ball is pressed against specific surfaces of the ball screw grooves without regard the moving direction of the nut member. Conventionally, however, the ball screw grooves are entirely finished, so that the finishing work requires much labor, the operating efficiency is poor, and the manufacturing cost is high.
Accordingly, the object of this invention is to provide an actuator device of which a screw shaft and a nut member can be worked with ease.